An integrated circuit chip package generally includes an integrated circuit lying on a chip support pad. Wire bonds connect the IC to a lead frame. A substance, such as plastic, encapsulates the structure. The plastic small outline J lead (POSJ) package is one exemplary example. For application, one technique uses reflow solder to surface mount the IC package to a printed circuit board.
As the industry moves towards thinner packages and packages containing chips of larger size for higher volumetric packaging, new packaging techniques are evolving. One such technique is the lead over chip package, (LOC). As described in the article entitled Volume Production of Unique Plastic Surface Mount Modules For The IBM 80-ns 1-Mbit DRAM Chip by Area Wire Bond Techniques by William C. Ward, published at the 38th ECC in IEEE 1988, pages 552-557, this technique disposes a lead frame over the active area of a semiconductor integrated circuit chip. Adhesive insulating tape attaches the lead frame to the chip. Wire bonds connect the chip to the centrally disposed power supply busses. And, wire bonds jump over the power supply busses to connect the chip to conductive lead fingers. No chip support pad is required.
Concerns exist about potential delaminating between the molding compound used to encapsulate the LOC package and the integrated circuit chip. Delamination of mold compound can create a significant problem resulting in plastic package cracking during surface mounting the LOC package to a printed circuit board. During the reflow solder process, where the temperature may range between about 215C to 260C, the heat generated by the process can enhance the state of thermal mismatch between dissimilar materials in the IC package and create high stresses. Moisture that may be present in the encapsulating mold compound is converted to steam. The steam pressure builds at a large continuous surface, such as the die backside in a LOC package. The steam pressure can be such that it delaminates the surface of the IC and the surface of the encapsulating material. The loss of adhesion under such conditions causes a high stress concentration in the encapsulant at the corner of the IC. This often leads to package cracking. In order to avoid this problem, "dry packing" is typically required.
It is an object of this invention to provide a semiconductor device package, such as the LOC package, that is resistant to cracking during surface mount. It is the further object of this invention to provide a method of promoting adhesion in a LOC package.
Other objects and benefits of this invention will be apparent to those skilled in the art, based upon the description to follow herein.